It seems to me that the car is not getting the same fuel economy that it did when I acquired it. I used to be able to get 400 miles from a tank of highway driving, but I haven't seen my trip meter pass 400 miles on a single tank of gas in over a year.

I live close to work, but frequently take trips > 100 miles during the weekends, so the majority of my driving is highway driving.

With gas prices going up this summer, I've been wondering if there is any kind of service I could perform on the car that would likely improve its fuel efficiency.

6 Answers
6

Keep it tuned up: for spark plugs and wires, go with the manufacturer recommended brands/default ranges. Additionally, you'll want to make sure you change the oil regularly, as sludgy oil will rob your motor of efficiency. Finally, a clean air and fuel filter will also go a long way to imrpoving fuel economy, as your motor doesn't have to work as hard to get the air and fuel it needs, respectively.

Your car requires Premium fuel, use it. Failure to use 90+ on your Acura actually will negatively affect much more than mileage, other components designed around and relying on prolonged use of the higher octane fuel are going to cause fuel economy issues as time wears on.

Keep your transmission fluid changed and clean. Waiting until the fluid is spent will not go well for you since less than half of the fluid comes out in a change. If the fluid is spent (very dirty) then you have added expense of a case flush to get things back on the right path.

Tires Matter. Use good tires. Cheap tires can be much noiser, wear more quickly and also not be designed for optimum fuel economy. See what the manufacturer recommends and stick with them. If you have changed to some sort of custom wheel/tire combination then fuel economy like you enjoyed on stock setup may be impacted. Proper inflation and rotation are also important.

Additonally:

Mileage will vary with seasons - you should be prepared for your mileage to change in different seasons, especially in many larger metropolitan areas where fuel formulations are changed to meet EPA restrictions and recommendations.

Your driving habits may be the greatest factor - being late for appointments, becoming more comfortable with the limits of your car, wanting to have more aggressive "fun"... they all come at a cost if you are using your car as the great equalizer

@wesanyer:I see you chose to edit my original text. I won't roll the edit back this time, but perhaps in the future try adding comments rather than modifying the source text would be a good idea, this preserves the text and thought process of the original author. If you must edit the original text in a format such as this resource then that edit should be made obvious so that any errata you add in will not negatively (or positively) impact the original author.
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rwheadonMar 8 '12 at 16:51

Some good points have been given already especially as far correct spark plugs and correct fuel go. Some other points, unfortunately, I can only describe as regurgitated 'car-care' sales drivel. I'm almost surprised that no one has mentioned that you should wax your car more often.

Here is a list of things that are not worth your money and time:

Changing oil more regularly that specified by your manufacturer. They knew about sludge formation when they specified the intervals, so that wont be issue. Do you really think that any change in fuel consumption due to slight changes in friction will cover the cost of the extra fuel?

Air filter. Clogged air filters are an obstruction to airflow and reduce maximum power output. This doesn't translate to reduced economy because, unless your highway driving is very different to mine, you probably cruise with a much more extreme air intake restriction already. It's called a throttle valve. If tomorrow you replaced your clogged air filter with the best flowing air filter you can find, you may gain some power, but to cruise at the same speed you would have your throttle slightly more closed in order to achieve the same manifold pressure, and thus the same output torque as required for cruise. No gain in economy. Here's a source: http://www.fueleconomy.gov/feg/pdfs/air_filter_effects_02_26_2009.pdf .

Fuel filter. Replace this as specified in your manual, as a blocked filter may case lean burning in wide open throttle, resulting in possible engine damage. However, if the filter was flowing so badly to affect fuel mixture at cruise (where something like 30hp is required) your engine would misfire at full throttle. Extra consumption due to extra fuel pump work is not going to be measurable; it'll still be drawing powerless than your headlights.

Tyres. Yes some tires are more efficient, but please do yourself a favour before dropping any money on new tyres and calculate how long at will take for the savings in fuel consumption to add up to the extra money you pay. I suspect that the answer will be longer than the service life of the tyre.

Now to address possible reasons for your increase in mileage:

Some, if not most, of your loss of mileage is from irreversible engine wear. I'm sure you already know this. You've lost some compression due to piston ring and valve seat wear. You wont get that back.

There are also some losses in mileage that you can recover, but diagnosing the sources of the losses might be quite labour intensive, so it will probably only be worth doing if you do the diagnosis yourself. Buying an OBDII scanner is a good start.

Here are the first things I would be looking at:

Oxygen sensor. This is the sensor that the ECU uses to determine that the fuel/air ratio it thinks it is supplying to the engine is actually being supplied. It is usually the ECUs only feedback about actual combustion. As they age oxygen sensors slow down in their response time and eventually the ECU will throw a Check Engine Light, but not until a significant drop in performance of the sensor has already occurred. There may be some sense in replacing an old O2 sensor before the CEL is thrown, however it may be a good idea to check its performance first. Here is a good resource on that: http://www.autodiagnosticsandpublishing.com/feature/o2-sensor-testing.htm.

Injector cleaning. Injectors slowly clog up and their flow rate is reduced with time. Unfortunately not all of them clog up at the same rate, causing some cylinders to run leaner that others. Because of the way oxygen sensors work even the slightest lean condition on one cylinder will cause the sensor to read lean, causing the ECU to run all of the other cylinders rich in compensation. Some people have had luck improving uneven injector delivery problems with concentrated over-the-counter fuel injector cleaner in the fuel tank. Your (figurative) mileage may vary, but this may be a good place to start before delving into deeper diagnosis.

Also: Cruise speed is probably the biggest change to your driving behaviour that you can make. I almost doubled my mileage when I had to limp home 400km with almost no effective headlights after a nighttime kangaroo collision.

There is no need to replace the wheel bearings, unless you have to. If alignment works fine, car tracks well, and there is no obvious signs of bearing deterioration (such as noise at high speeds), why replace parts that work perfectly well?
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theUgMay 31 '12 at 15:05

Since no one's mentioned it, try replacing the PCV valve. You probably have more fuel blowby in a higher mileage car since the rings are probably more worn, and it's the PCV valve which let's that fuel and vapours be recycled and reburned by the engine.

The PCV valve is part of the positive crankcase ventilation system, which reroutes unburned gases, or blow-by, from the crankcase to the intake manifold and back to the engine, where they can be reburned in the cylinders. This process cuts the amount of pollution released into the environment. It also increases fuel economy because unburned fuel in the blow-by is consumed the second time around.

In today’s heavily computer controlled and adjusted vehicles a malfunctioning PCV valve can cause non optimal tuning characteristics via your engine’s management system that can contribute to reduced power and reduced fuel economy. Thus it is very important to check you PCV valve and replace it on a high mileage vehicle as it is often ignored.

Lets say your car is upto date with its servicing, and maintenance too. Your car is now older and more worn then it was when you first got it. It can only be less efficient in itself. Now you can either spend money on changing all manner of bits and pieces on your car or spend your money on a new fuel efficient car.